Transient Electromagnetic Resistivity Survey at the Geysir Geothermal Field South Iceland (original) (raw)
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2017
Geothermal exploration involves geology, geochemistry and geophysics. In geophysical exploration, resistivity surveying plays the most important role in delineating the reservoir. The parameters that control the geothermal system show a strong response to electrical resistivity. The resistivity methods that are mostly used in geothermal exploration in Iceland are TEM (Transient ElectroMagnetics) and MT (MagnetoTellurics). The application of these methods is discussed in this report together with an example from the Eyjafjördur low-temperature area in N-Iceland. The resulting resistivity cross-sections and resistivity depth slices show a shallow lying low-resistivity layer and a deep lying low-resistivity anomaly towards the end of the cross-sections. The result of this work has been compared with results from Flóvenz and Karlsdóttir (2000) which interpreted TEM data from the same area. The results are also compared with borehole data and stratigraphy.
2012
Measuring the electrical resistivity,ρ, of the subsurface is the most powerful geophysical prospecting method in high temperature geothermal exploration and the main method used in delineating geothermal resources. Resistivity surveys using TEM and MT methods have been done in the Theistareykir geothermal area since 2004 and in this work; sixteen TEM and MT soundings on two profiles have been jointly inverted based on 1D models. Cross sections and iso-resistivity maps of the joint inversion models of the TEM and MT data from Theistareykir geothermal area reveal features similar to other high temperature areas in Iceland with a resistive zone reflecting unaltered rock formations, shallow conductive cap and a resistive core reflecting chlorite-epidote alteration. Two deep conductors (<10 Ωm) possibly related to the heat source have been interpreted north of Bóndhólsskarð and north east of Stórihver. Good correlation between the subsurface resistivity and hydrothermal alteration in ...
In Eyjafjordur in N-Iceland several small hot springs were found. Drilling at the various hot springs has shown that the geothermal activity is mainly connected to fractures in the basaltic pile of very low permeability but there are also indications of some near horizontal flow. TEM resistivity soundings have been used to create a resistivity image of the basaltic pile in the area to improve the older resistivity picture obtained by Schlumberger soundings. The TEM soundings were initially inverted to give a layered resistivity model without considering any other geological knowledge. This was done to avoid biasing during interpretation. The result shows several layers of high and low resistivity with southerly dip. This pattern was not resolved in the previous Schlumberger soundings. An additional local low-resistivity zone is observed. Comparison of the TEM result with borehole-and surface data shows that the resistivity layering coincides with the lithological layering. The low r...
Resistivity is one of the most variable physical property of materials and has proven to be most useful in the search for geothermal resources. To understand the geothermal significance of the distribution of resistivity, a review of the parameters affecting resistivity in geothermal systems was conducted in this work. The conductive clay products of hydrothermal alteration are the most common cause of low resistivity in the zone above the reservoir. Correlations between alteration type and resistivity can extend further to enable better predictions of the reservoir temperature distribution from surface geophysical measurements. Transient electromagnetics (TEM) are used for measuring shallow structures and Magnetotellurics (MT) are used for probing deeper. The joint inversion of TEM and MT data has proven to be useful in solving the static shift problem resulting from near surface resistivity inhomogeneities. The MT apparent resistivity can be affected especially in volcanic areas and shifted by a multiplicative factor which is frequency independent.
2009
Resistivity-logs from boreholes are compared to a previously interpreted resistivity model from TEM and MT electromagnetic soundings in the high temperature geothermal field at Hellisheidi, which is a part of the Hengill volcano system, SW-Iceland. Previous studies of resistivity from the geothermal system in the Hengill area have shown a low resistivity cap on the outer margins of the reservoir underlain by higher resistivity and further down a deep conductor which is below the depth of the present study. The resistivity pattern was explained with conduction of temperature alteration minerals which remained when a geothermal system cooled, thus the resistivity could be regarded as a “maximum thermometer”. A low resistivity layer is also present in the well-logs in this study. On the other hand the depth and extensions of the low resistivity zones do not coincide completely with the interpreted resistivity from the surface measurements. Farther down there are less similarities betwe...
2012
Geophysical methods are most useful in extracting subsurface information. Which geophysical method should be used to characterize a site depends on what information one needs. Resistivity has a direct relationship with the subsurface rock temperature. TEM and MT are very cost effective methods in a subsurface resistivity study. Although the MT method is very efficient at getting information down to the mantle, it suffers a static shift problem. Joint interpretation of TEM and MT data removes the static shift from MT data. Geologically, Námafjall is a very important area in Iceland. Tectonically it is related to Krafla volcano. It was formed in subglacial eruptions during the last glaciation and has undergone cooling. In this study sixteen TEM and MT soundings were used to evaluate the subsurface resistivity. At shallow depth there is a low-resistivity layer having resistivity less than 10 Ωm below Námafjall ridge. On the surface this area coincides with surface geothermal manifestat...
Resistivity methods in geothermal prospecting in Iceland
Surveys in geophysics, 1994
Resistivity techniques have been used successfully to identify and delineate geothermal resources in Iceland. The most frequently used techniques include Schlumberger, central loop TEM and head-on profiling. Geothermal systems in Iceland are located both within and outsite the active volcanic regions. Outsite the active volcanic regions the temperature in the upper most kilometer of the geothermal systems is below 150°C whereas the temperature in the geothermal fields within the active volcanic regions exceeds 200 ° C. The resistivity of the rock in geothermal fields located outside the active volcanic regions ranges from about 10 ~m to some hundreds of f~m, and are characterized by considerably lower resistivity than of the surrounding rocks. Most of the geothermal systems within the active volcanic regions, show common resistivity structure with low resistivity of 1-5 f~m surrounding an inner core of higher resistivity. This increasing resistivity with depth is associated with a change in the conduction mechanism, from interface conduction to electrolyte conduction due to a change in alteration minerals at about 240 ° C. Examples of resistivity surveys of geothermal fields from both outsite and within the active volcanic regions are discussed. shows the main features of the Icelandic geology. The Mid-Atlantic ridge transects Iceland from southwest to northeast along the active zones of rifting and volcanism. These zones are characterized by many active central volcanos with
The resistivity structure of high-temperature geothermal systems in Iceland
Proceedings of the World …, 2000
Electrical and electro-magnetic methods have been used extensively to identify and delineate high-temperature geothermal reservoirs in Iceland. All hightemperature systems, within the basaltic crust in Iceland, have a similar resistivity structure, characterized by a low resistivity cap at the outer margins of the reservoir, underlain by a more resistive core towards the inner part. This is found in fresh-water systems as well as brine systems, with the same character but lower resistivities in the brine systems. Comparison of this resistivity structure with data from wells shows a good correlation with alteration mineralogy. The low resistivity in the low-resistivity cap is dominated by conductive minerals in the smectite-zeolite zone in the temperature range of 100-220ºC. At temperatures 220-240ºC zeolites disappear and the smectite is gradually replaced by the resistive chlorite. At temperatures exceeding 250ºC chlorite and epidote are the dominant minerals and the resistivity is probably dominated by the pore fluid conduction in the high-resistivity core. The important consequence of this is that the observed resistivity structure can be interpreted in terms of temperature distribution. A similar resistivity structure is to be expected in acidic rocks. Due to different alteration mineralogy, however, the transition from the conductive cap to the more resistive core presumably occurs at temperatures lower than 200°C.
2014
A brief summary of the importance of the electrical resistivity of rocks in geothermal exploration is given as well as some of its is main relationships with some of the characteristics of geothermal reservoirs. Special emphasis is on the resistivity of rocks in high temperature geothermal fields where the dominant factor is the hydrothermal mineral alteration and how it relates to temperature. This review is to a great extent based on experience and work done through the years by Iceland GeoSurvey and its predecessor the NEA. In this review it should be pointed out that an overview of geophysical exploration for geothermal resources was given in a UNU GTP publication (Hersir and Bjornsson, 1991). Other important sources here are two more recent articles from the World Geothermal Congresses (Arnason et. al., 2000 and Flovenz et al., 2005).
Resistivity Survey in the Alid Geothermal Area, Eritrea, a Joint Interpretation of TEM and MT Data
… Congress 2010, Bali, …, 2010
A geophysical survey of the Mt. Alid geothermal area in Eritrea was performed in late 2008. The aim of the survey was to delineate the geothermal reservoir by the use of Transient Electro Magnetism (TEM) and Magnetotelluric (MT) resistivity soundings. The original plan was to cover the area of Mt. Alid and its nearest vicinity. The terrain, however, was very difficult for vehicles and bikes used for transportation of the equipment and inaccessible at times. The area to the west and southwest of the mountain was covered as well as the top of Mt. Alid, within the allotted timeframe. A total of 67 TEM and 52 MT soundings were performed in 24 days. The TEM and MT soundings were performed in the same place. The TEM revealed the shallow resistivity structure and MT the deeper part. The TEM data were also used for static shift correction in the MT data through a joined 1-D inversion of the two data sets. A SW-NE lineament is detected at a depth interval of 500 metres to 2 km, and interpreted as a transform fault intersecting the geothermal reservoir which most likely controls the main upflow from the reservoir. The surface on the southwest flanks of Mt. Alid has an area with exceptional vegetation in comparison to the barren dry volcanic landscape around. This indicates moisture and water in the ground that could be explained by steam flowing up from the geothermal reservoir.